Page 116 - Microtectonics

104 4 · Foliations, Lineations and Lattice Preferred Orientation
Theoretically, it should be possible to use LPO patterns sections through an ODF, and inverse pole diagrams where
as a source of information on the six parameters men- the crystal axes are taken as a reference frame and the
tioned above. However, our understanding of the devel- orientation of the lineation in the rock with respect to
opment of LPO is unfortunately still sketchy. Most success- this frame is plotted for each grain (Fig. 4.40a). ODF can
ful has been the application of LPO patterns with mono- also be useful if the preferred orientation of a certain crys-
clinic symmetry to determine sense of shear (Sect. 4.4.4.3). tal direction that is of interest cannot be measured di-
The study of the development of LPO proceeds through rectly; from an ODF it is always possible to calculate such
several angles of approach. Observation of natural LPO orientations.
patterns and comparison with known temperature, strain Since ODF are difficult to read, stereograms are most
geometry and vorticity of the progressive deformation can commonly used, either directly plotted from measured
give an indication of the influence of these parameters on data or derived from the ODF through calculation (Schmid
LPO development. However, in natural LPO, the deforma- and Casey 1986). LPO patterns in stereograms can ap-
tion history is usually unknown and may have been more pear as point maxima or as small- or great circle girdles.
complex than is assumed; early parts of the development In complex LPO patterns, the girdles are connected with
are most likely erased. Slip systems may be identified by each other to form crossed girdles of either Type I or
observation of lattice defects in naturally deformed crys- Type II (Lister 1977; Fig. 4.40b). Cleft girdles (actually
tals by TEM (Blacic and Christie 1984; Hobbs 1985). How- small circles) are formed in flattening strain. If a preferred
ever, lattice defects in natural deformed rocks may be orientation is present, but the pattern is vague, pole-free
formed late, after the LPO was developed (White 1979a; areas can be distinguished. In order to enhance visibility
Ord and Christie 1984). Theoretical and numerical mod- of girdles and maxima, LPO patterns are usually con-
elling of fabric development using a pre-set choice of slip toured. Contours can be used to derive a fabric skeleton, a
systems have been very successful in modelling LPO pat- pattern of lines connecting the crests of the contour dia-
terns (Etchecopar 1977; Lister 1977; Lister and Price 1978; gram (Fig. 4.40b).
Lister et al. 1978; Lister and Paterson 1979; Lister and Hobbs LPO patterns are interpreted in terms of their internal
1980; Etchecopar and Vasseur 1987; Jessell 1988b), but theo- and external asymmetry. Internal asymmetry is defined
retical studies suffer from assumptions that may be wrong by the shape of the pattern itself; external asymmetry is
and simplifications necessary to operate computer mod- determined with respect to a reference frame (Sects. 2.4,
els. Furthermore, only monomineralic aggregates have 5.6.1); lacking other possibilities, fabric elements such as
been simulated, while most of the interesting fabrics in foliations and lineations in a rock are normally used as a
rocks occur in polymineralic aggregates. The most suc- reference frame, notably those that are thought to have
cessful, but possibly also most laborious approach to study formed at the same time as the LPO. For briefness, such
LPO development, is experimental deformation of rocks reference foliations and lineations are given in this chap-
at high pressure and temperature and subsequent analy- ter as S and L . r
r
sis of the LPO patterns in deformed samples, in combina- In stereograms, standard presentation of LPO patterns
tion with TEM analysis of lattice defects (Green et al. 1970; is with the Y-direction of finite strain vertical and the
Tullis et al. 1973; Dell’Angelo and Tullis 1989). X- and Z-directions along the EW and NS axes (Fig. 4.41).
This implies that a corresponding foliation and lineation
4.4.3 are presented in the diagram as an E-W-trending vertical
Presentation of LPO Data plane (S ) and horizontal line respectively, the latter indi-
r
cated by dots on the circle (L ; Fig. 4.40b). L is usually an
r
r
The orientation of a crystal in a reference frame is only aggregate or grain lineation. This orientation shows the
completely defined if the orientation of three crystal axes symmetry of most LPO patterns advantageously.
is known; this means that three numbers are needed to It is commonly useful to show which grains in an ag-
represent the orientation of a single crystal in a reference gregate have a particular orientation. The distribution of
frame. However, if an LPO is to be presented in this way, it grains with particular orientations can be given in a map
can only be done as points in a three-dimensional dia- of the sample under consideration, known as an AVA dia-
gram. Such a diagram is known as an orientation distri- gram (German: ‘Achsenverteilungsanalyse’ – analysis of
bution function diagram or ODF (Fig. 4.40a). In practice, orientation of axes; Sander 1950; Heilbronner-Panozzo and
it may be difficult for the inexperienced to read such dia- Pauli 1993). In practice, AVA diagrams are made for the LPO
grams. Geologists usually rely on polar diagrams such as pattern of a single crystal axis, such as c-axes. An AVA dia-
stereograms to plot the orientation of crystals (Fig. 4.40a); gram can be presented by plunge direction of c-axes for
however, these are only useful if just one crystallographic each grain, presented as lines (Fig. 4.24) or, more ad-
direction, such as the c-axis of quartz, is plotted. In this vanced, by colours representing different orientations. AVA
way, only part of the LPO pattern of a crystalline aggre- can be of great help for the interpretation of LPO pat-
gate is presented. Other methods of presentation are cross- terns and of the way in which they develop (Sect. 10.3).